Heart disease strikes millions of Americans and represents the leading cause of death. Approximately one million people die each year in the United States from heart and vascular disease (Heart and Stroke Facts, Dallas: American Heart Association, 1993). Impaired coronary blood flow, either partial or total occlusion, results in myocardial ischemia. At least half the number of patients afflicted with myocardial ischemia suffer from further coronary pathology where heart tissue dies from anoxia or lack of oxygen. This pathology, myocardial infarction, can further result in other manifestations of coronary disease including cardiac arrhythmias (Downey and Mullane, Methods for protecting tissues and organs from ischemic damage U.S. Pat. No. 5,573,772; which is hereby incorporated by reference in its entirety herein).
Damage to myocardial tissue from ischemia, decreased oxygenated blood flow to muscle tissue, can be reduced by preconditioning. Brief periods (5-10 minutes) of ischemia have been shown to precondition against more prolonged periods of ischemia. Such preconditioning appears to provide protection against greater pathologic effects on myocardial tissue that arise from ischemia compared with tissues not preconditioned. Additional pathology results from reperfusion injury which occurs following a return of blood flow to previously ischemic myocardium. Preconditioning therefore, may serve as a form of preventive therapy to those patients presenting with impaired coronary vascular disease.
Opioid receptor activation has been implicated to elicit a protective effect during situations of stress produced by hypoxia, ischemia, cold or acidic environments (for example, see: Arrigo-Reina R, Ferri S. Evidence for an involvement of the opioid peptidergic system in the reaction to stressful condition. Eur J Pharmacol 1980:64:85-88; and Mayfield, K. P., D'Alecy L. G. Role of endogenous opioid peptides in the acute adaptation to hypoxia. Brain Res. 1992;582:226-231) The delta (.delta.) opioid receptor has been demonstrated to play a major role in this protection. Chien, S. et al., J Thorac Cardiovasc Surg (1994) 107:964-967; and Mayfield, K. P., et al., J Pharmacol Exp Ther (1994) 268:683-688 and J Pharmacol Exp Ther (1994) 268:74-77; and Schultz, J. J. et al., J Mol Cell Cardiol (1997) (in press) showed using DPDPE (selective delta-1 (.delta..sub.1) opioid receptor agonist) and BNTX (selective delta-1 (.delta..sub.1) opioid receptor antagonist) that the delta-1 (.delta..sub.1) opioid receptor mediated the adaptation or increased survival time of mice to hypoxic environments. Furthermore, Chien, S. et al., demonstrated that the time prior to organ transplantation was increased significantly from a 6 hour window to a 48 hour window following the administration of a synthetic delta (.delta.) opioid receptor agonist, DADLE. These delta (.delta.) opioid receptors have been shown to be involved in the cardioprotective effect of ischemic PC in the intact rat heart. Schultz, J. J., et al., J Mol Cell Cardiol (1997) (in press).
Specifically, recent studies have demonstrated that opioid receptors are involved in ischemic preconditioning (PC) in an intact rat model (Schultz, J. J. et al., Am J Physiol (1995) 268:H2157-H2161 and Schultz, J. J. et al., Circ Res (1996) 78:1100-1104). Additional studies showed that naloxone, a non-selective opioid receptor antagonist, blocked the cardioprotection afforded by brief periods of ischemia (Schultz, J. J. et al., Am J Physiol (1995) 268:H2157-H2161). Furthermore, a non-selective opioid receptor agonist, morphine, has been found to mimic the cardioprotective effect of ischemic PC and that this morphine-induced myocardial protection was antagonized by naloxone (Schultz, J. J. et al., Circ Res (1996) 78:1100-1104). Subsequent research has provided evidence that opioid receptors are involved in ischemic PC in the rabbit heart, Chien, GL. and Van Winkle, D. M., J Mol Cell Cardiol (1996) 28:1895-1990. Other recent studies demonstrated that morphine was cardioprotective in the isolated rabbit heart, Miki, T. and Downey, J., J Mol Cell Cardiol (1996) 28:A187.
Myocardial binding studies have shown that 8 and K-opioid receptors are present on ventricular myocytes of the rat (Krumins, S. A., et al., Biochem Biophys Res Comm (1985) 127:120-128; Ventura, C., et al., Biochem Biophys Acta (1989) 987:69-74; Tai, K. K., et al., J Mol Cell Cardiol (1991) 23:1297-1302; Zhang, W-M., et al., (1996) J Mol Cell Cardiol 28: 1547-1554; Zimlichman, R., et al., (1996) Circulation 93: 1020-1025). Also, .delta.- and .kappa.-opioid receptors have been demonstrated on the ventricular cardiac sarcolemma of the rat (Ventura and colleagues (1989)). Similar studies have revealed .delta.- and .kappa.-, but not .mu.-opioid receptors on rat atrial and ventricular tissue (Krumins et al. (1985)). Developmental studies showed the presence of .delta.- and .kappa.-opioid receptors in adult rat heart; whereas, only .mu.- and .kappa.-opioid receptors were present in neonatal rat hearts (Zimlichman et al. (1996)). Additional evidence has been presented that .delta.- and .kappa.-opioid receptors are present on canine cardiac sarcolemma and inhibit adenylate cyclase activity via activation of G.sub.i proteins (Mura and Niroomand (1996)). However, the role of the specific .delta.-opioid receptor subtype (.delta..sub.1 and .delta..sub.2) as well as a role for .mu.- and .kappa.-opioid receptors in the cardioprotective effect of ischemic PC remains unknown.
Liu et al. (Am. J. Physiol. 1992: 263:(Heart Circ. Physiol. 32):H1107-H1112) has showed that ischemic PC protected against myocardial infarction and that this effect was mediated by adenosine A1 receptors in the rabbit. Gross and Auchampach (Circ. Res. 1992:70:223-233) were the first to demonstrate that preconditioning was mediated through the ATP-sensitive potassium (K.sub.ATP) channel in the canine heart. In addition, G.sub.i -proteins (Lasley et al. J. Mol. Cell Cardiol. 1993:25:815-821; Thornton et al. J Mol. Cell Cardiol. 1993:25:311-320) protein kinase C (PKC) (Ytrehus et al. Am. J Physiol 1994: 266:(Heart Circ. Physiol.):H1145-H1152), muscarinic receptors (Yao et al. Am. J. Physiol. 1993: 264:(Heart Circ. Physiol. 34):H2221-H2225; Yao et al. Circ. Res. 1993:73:1193-1201), and the Na.sup.+ /H.sup.+ exchanger (Bugge et al. Basic Res. Cardiol. 1996:91:203-209; Piper et al. Basic Res. Cardiol. 1996:91:191-202; Rohmann et al. Cardiovasc. Res. 1995:30:945-951) have been implicated in the mechanism(s) of ischemic PC. The two prominent, potential cardioprotective mechanisms, the adenosine A1 receptor and the K.sub.ATP channel, have also been investigated in the rat heart; however, Liu et al. (Am. J. Physiol. 1992: 263:(Heart Circ. Physiol. 32):H1107-H1112) found that neither mediator appeared to be responsible for ischemic PC in this species. Recently, the K.sub.ATP channel has been shown to mediate ischemic PC in the intact rat model of myocardial infarction (Qian et al. Am. J. Physiol.1996: 271:(Heart Circ. Physiol. 40):H23-H28; Schultz et al. J. Mol. Cell Cardiol. 1997:29:1055-1060; Schultz et al. Am. J. Physiol. 1997: 272:(Heart Circ. Physiol. 41):H2607-2615). Furthermore, stimulation of certain second messengers such as PKC (Li et al. Am. J. Physiol. 1995: 268:(Heart Circ. Physiol. 37):H426-431; Speechly-Dick et al. Circ. Res. 1994:75:586-590), heat stress proteins (Cox et al. In: Herz A., ed. Handbook of Experimental Pharmacology: Opioids I. New York: Springer-Verlag; 1993: 143-188) and muscarinic receptor activation (Qian et al. Am. J. Physiol. 1996: 271:(Heart Circ. Physiol. 40):H23-H28; Richard et al. Br. J Pharmacol. 1995:115:1532-1538) have been proposed to reduce myocardial necrosis in intact rats.
Recently, the Applicants have demonstrated that the opioid receptor system is involved in eliciting the cardioprotective effect of ischemic PC in the rat (Schultz et al. J. Mol. Cell Cardiot. 1997:29:2187-2195; Schultz et al. J. Mol. Cell Cardiol. 1997:29:1355-1362; Schultz et al. Circ. Res. 1996:78:1100-1104; Schultz et al. J. Mol. Cell Cardiol. 1997:29:A200; Schultz et al. Am. J. Physiol. 1995: 268:(Heart Circ. Physiol. 37):H2157-H2161). Applicants have shown that .delta.-opioid receptors, most notably the .delta..sub.1 -opioid receptor, mediate the cardioprotective effect of ischemic PC (Schultz et at. J. Mol. Cell Cardiol. 1997:29:2187-2195; Schultz et al. J. Mol. Cell Cardiol. 1997:29:A200). A number of investigators have provided evidence that .LAMBDA.-opioid receptors exist on cardiac myocytes (Krumins et al. Biochem. Biophys. Res. Comm. 1985:127:120-128; Ventura et al. Biochim. Biophys. Acta. 1989:987:69-74; Ventura et al. Circ. Res. 1992:70:66-81; Zimlichman et al. Circulation 1996:93:1020-1025). In addition, Wittert and colleagues (Wittert et al. Biochem. Biophys. Res. Comm. 1996:218:877-881) determined the distribution of expression of the mu (.mu.)-, kappa (.kappa.)- and .delta.-opioid receptors in peripheral tissue of the rat and found that .delta.-receptor transcripts were predominantly detected in the heart; whereas, a weak .mu.- and no .kappa.-receptor transcripts were measured. Therefore, we tested the hypothesis that stimulating .delta..sub.1 -opioid receptors would reduce myocardial infarct size and that this opioid receptor-mediated cardioprotection involved a mechanism similar to that observed with ischemic PC in the rat heart (Schultz et al. Am. J. Physiol. 1998: 274:(Heart Circ. Physiol. 43):H909-H914).